One type of present day small aircraft muffler currently used has a series of cones that are tack welded to small diameter rods. This assembly is placed within a housing that has an inlet and an outlet. Although this muffler design works well to muffle noise, it has a tendency to fail. Vibration and heat often cause one or more of the cones to break free of the welds. In many cases, the outlet side cone will invert and become lodged in the outlet of the muffler causing immediate power loss due to extreme back pressure in the introduction system. If this power loss occurs during takeoff, a crash is highly likely. The cones will also break loose in the center of the muffler, which will cause the exhaust gases to be directed toward the muffler housing. This will cause flame impingement that will lead to fatigue in the metal and eventual failure. This failure will often release carbon monoxide into the cabin heating system because mufflers are often used as heat exchangers in such systems. Over the years many different muffler designs have been tested and tried. One such design is the so-called "plug" muffler. A plug muffler works by placing a solid plug in the center of an exhaust tube. Holes are provided about the circumference of the exhaust tube. The exhaust enters the inlet of the tube and is forced outward through the holes because it cannot pass through the plug. A corresponding set of holes is provided on the outlet end of the muffler to permit the exhaust to reenter the exhaust tube where it can exit to the atmosphere. This movement through the holes acts to attenuate the sound emitted from the muffler.
An example of a plug muffler design is U.S. Pat. No. 3,997,002 to Baker et al. This muffler was designed to be used in a kit form where the exhaust gas conduit could be easily removed from the shell and replaced. In practice, this muffler has several problems. The heat experienced within the muffler often caused the tube to gall and expand. Many times, the plug would come loose and open the exhaust tube, thereby reducing the effectiveness of the muffler. Once the tube expands or galls, it is impossible to remove it from the housing. Often these tubes must be cut from the housing, ruining the muffler. The Baker et al. design uses simple holes drilled into the cylindrical exhaust tube.
U.S. Pat. No. 2,828,830 to Clark shows a plug muffler having directional hoods formed over the holes. These hoods assist the airflow through the housing. Although these hoods do make some improvement, they do nothing but rise above the tube surface. Moreover, the hoods can act as impediments to the airflow as it reenters the exhaust tube, which can cause the exhaust gases to "search" for an opening, possibly increasing the back pressure to the induction system.